Abstract Chilling injury of vegetative and generative buds, bark and wood during fall and winter seasons are mainly limiting factors for productivity of fruits. Selecting and expanding cultivars for more yield and quality without attention to the rate of their frost resistance are impossible. In this research, the rate of relative resistance of some peach and nectarine cultivars at a sudden downfall of temperature was analyzed in vivo weather conditions during the 2008 winter season in Mashhad. The meteorology station of the studied area (Golmakan station) registered -22°C for the minimum temperature of this year on 13th January, which was a little different from the weather of Mashhad. This test is performed as factorials in the form of a randomized complete block design with three replications. Relative ionic leakage is used on reproductive and vegetative buds, annual and biennial wood in peach cultivars (J.H.Hale, Elberta, Redhaven, Mashhad Sorkhosepfide and Mashhad Ghermez) and nectarine cultivars (Indipendens, Qiuota, Nectared 6 and Stargold). Between studied peach and nectarine cultivars, Mashhad Sorkhosephid and Mashhad Ghermez cultivars, which are cultivars native to Iran, were the most resistant reproductive buds that did not have a significant difference in comparison with other peach and nectarine cultivars with the exclusion of Indipendense cultivar. The most sensitive reproductive buds were nectarine Indipendense cultivar and the most resistant vegetative buds were Mashhad Sorkhosephid and Redhaven in peach cultivars and Nectared 6 and Qiuta cultivars in nectarine cultivars. The relative ionic leakage of annual wood in peach cultivars was more than nectarine cultivars. But the relative ionic leakage between peach cultivars was not significant and Nectared 6 cultivar did not have any difference compared to peach cultivars. The \\\"Sorkhosephid\\\" peach cultivar of Mashhad had more solution carbohydrates in generative buds and annual wood (93.33 and 136.50 mg/lit, respectively) than other studied cultivars. Results of sugar rate followed relative ionic leakage and visual observations and showed increasing sugar and decreasing ionic leakage in organ caused lower their susceptibility to cold.

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